24 research outputs found
The Chandrasekhar Spitzer controversy and the (ir)relevance of distant interactions
We have done N-body simulations with N up to 106, with the aim to determine whether fluctuations in the force field of a globular cluster are caused by nearby or distant encounters. We find that distant encounters are insignificant, in agreement with Chandrasekhar's expectations, contrary to general opinion
HI imaging of Galaxy Clusters at z=0.2; a Pilot Survey of Abell 963 and Abell 2192
A pilot study with the powerful new backend of the Westerbork Synthesis Radio
Telescope (WSRT) of two galaxy clusters at z=0.2 has revealed neutral hydrogen
emission from 39 galaxies. The volume probed for each cluster is 1.7x10^4
Mpc^3, with spatial and velocity resolutions of 54x86 kpc^2 and 19.7 km/s,
covering both clusters and the large scale structure in which they are
embedded. The spatial distribution of the HI detected galaxies is very
different for the two clusters. In Abell 963, most of the gas-rich galaxies are
located to the northeast, at 1-3 Mpc from the cluster center in projection.
Their velocities are slightly redshifted with respect to the cluster mean. This
could be a gas-rich group falling in from the front. Abell 2192 is less massive
and more diffuse, with the gas rich galaxies more uniformly spread over a large
region around the cluster. The HI masses of the detected galaxies range from
5x10^9 to 4x10^10 Msun. Some HI-rich galaxies are spatially resolved and
rudimentary rotation curves are derived, showing the prospect for Tully-Fisher
studies of different galaxy populations in these environments. Only one galaxy
is detected within a 1 Mpc radius from the center of the Butcher-Oemler cluster
Abell 963, and none of the blue B-O galaxies which are all located within the
central Mpc. The HI detected galaxies outside the central Mpc are of similar
colour and magnitude as the non-detected B-O galaxies, indicating that the blue
B-O galaxies are gas-poor compared to their counterparts in the field.Comment: 7 pages, 4 figures. to appear in the New Astronomy Review proceedings
of "The Fate of Gas in Galaxies", held in Dwingeloo, July 200
Westerbork Ultra-Deep Survey of HI at z=0.2
In this contribution, we present some preliminary observational results from
the completed ultra-deep survey of 21cm emission from neutral hydrogen at
redshifts z=0.164-0.224 with the Westerbork Synthesis Radio Telescope. In two
separate fields, a total of 160 individual galaxies has been detected in
neutral hydrogen, with HI masses varying from 1.1x10^9 to 4.0x10^10 Msun. The
largest galaxies are spatially resolved by the synthesized beam of 23x37
arcsec^2 while the velocity resolution of 19 km/s allowed the HI emission lines
to be well resolved. The large scale structure in the surveyed volume is traced
well in HI, apart from the highest density regions like the cores of galaxy
clusters. All significant HI detections have obvious or plausible optical
counterparts which are usually blue late-type galaxies that are UV-bright. One
of the observed fields contains a massive Butcher-Oemler cluster but none of
the associated blue galaxies has been detected in HI. The data suggest that the
lower-luminosity galaxies at z=0.2 are more gas-rich than galaxies of similar
luminosities at z=0, pending a careful analysis of the completeness near the
detection limit. Optical counterparts of the HI detected galaxies are mostly
located in the 'blue cloud' of the galaxy population although several galaxies
on the 'red sequence' are also detected in HI. These results hold great promise
for future deep 21cm surveys of neutral hydrogen with MeerKAT, APERTIF, ASKAP,
and ultimately the Square Kilometre Array.Comment: 10 pages, 9 figures, Proceedings of ISKAF2010 Science Meeting: A New
Golden Age for Radio Astronomy, June 10-14 2010, Assen, the Netherlands.
Edited by J. van Leeuwen. Movies of rendered rotating data cubes are
available at http://www.astro.rug.nl/~verheyen/BUDHIES/index.htm
A probabilistic approach to phase calibration: I. Effects of source structure on fringe-fitting
We propose a probabilistic framework for performing simultaneous estimation
of source structure and fringe-fitting parameters in Very Long Baseline
Interferometry (VLBI) observations. As a first step, we demonstrate this
technique through the analysis of synthetic short-duration Event Horizon
Telescope (EHT) observations of various geometric source models at 230 GHz, in
the presence of baseline-dependent thermal noise. We perform Bayesian parameter
estimation and model selection between the different source models to obtain
reliable uncertainty estimates and correlations between various source and
fringe-fitting related model parameters. We also compare the Bayesian
posteriors with those obtained using widely-used VLBI data reduction packages
such as CASA and AIPS, by fringe-fitting 200 Monte Carlo simulations of each
source model with different noise realisations, to obtain distributions of the
Maximum A Posteriori (MAP) estimates. We find that, in the presence of resolved
asymmetric source structure and a given array geometry, the traditional
practice of fringe-fitting with a point source model yields appreciable offsets
in the estimated phase residuals, potentially biasing or limiting the dynamic
range of the starting model used for self-calibration. Simultaneously
estimating the source structure earlier in the calibration process with formal
uncertainties improves the precision and accuracy of fringe-fitting and
establishes the potential of the available data especially when there is little
prior information. We also note the potential applications of this method to
astrometry and geodesy for specific science cases and the planned improvements
to the computational performance and analyses of more complex source
distributions.Comment: accepted for publication in MNRA
Extinction Curves, Distances, and Clumpiness of Diffuse Interstellar Dust Clouds
We present CCD photometry in UBVRI of several thousand Galactic field stars
in four large (>1 degree^2) regions centered on diffuse interstellar dust
clouds, commonly referred to as ``cirrus'' clouds (with optical depth A_V less
than unity). Our goal in studying these stars is to investigate the properties
of the cirrus clouds. A comparison of the observed stellar surface density
between on-cloud and off-cloud regions as a function of apparent magnitude in
each of the five bands effectively yields a measure of the extinction through
each cloud. For two of the cirrus clouds, this method is used to derive UBVRI
star counts-based extinction curves, and U-band counts are used to place
constraints on the cloud distance. The color distribution of stars and their
location in (U-B, B-V) and (B-V, V-I) color-color space are analyzed in order
to determine the amount of selective extinction (reddening) caused by the
cirrus. The color excesses, A_lambda-A_V, derived from stellar color histogram
offsets for the four clouds, are better fit by a reddening law that rises
steeply towards short wavelengths [R_V==A_V/E(B-V)<=2] than by the standard law
(R_V=3.1). This may be indicative of a higher-than-average abundance of small
dust grains relative to larger grains in diffuse cirrus clouds. The shape of
the counts-based effective extinction curve and a comparison of different
estimates of the dust optical depth (extinction optical depth derived from
background star counts/colors; emission optical depth derived from far infrared
measurements), are used to measure the degree of clumpiness in clouds. The set
of techniques explored in this paper can be readily adapted to the Sloan
Digital Sky Survey data set in order to carry out a systematic, large-scale
study of cirrus clouds.Comment: 22 pages, 13 figures (postscript, gif, jpg). Accepted for publication
in the Astronomical Journal, scheduled for the May 1999 issue. Full
resolution postscript versions of all figures are available at
http://www.ucolick.org/~arpad
Optical Spectroscopy of Galactic Cirrus Clouds: Extended Red Emission in the Diffuse Interstellar Medium
We present initial results from the first optical spectroscopic survey of
high latitude Galactic cirrus clouds. The observed shape of the cirrus spectrum
does not agree with that of scattered ambient Galactic starlight. This mismatch
can be explained by the presence of Extended Red Emission (ERE) in the diffuse
interstellar medium, as found in many other astronomical objects, probably
caused by photoluminescence of hydrocarbons. The integrated ERE intensity,
I_ERE \approx 1.2 x 10^{-5} erg s^{-1} cm^{-2} sr^{-1}, is roughly a third of
the scattered light intensity, consistent with recent color measurements of
diffuse Galactic light. The peak of the cirrus ERE (lambda_{0} \sim 6000 AA) is
shifted towards short (bluer) wavelengths compared to the ERE in sources
excited by intense ultraviolet radiation, such as HII regions (lambda_{0} sim
8000 AA); such a trend is seen in laboratory experiments on hydrogenated
amorphous carbon films.Comment: 7 pages, 2 figures. Accepted for publication in ApJ Letter
Overview of the coordinated ground-based observations of Titan during the Huygens mission
Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) radio telescope tracking of the Huygens signal at 2040 MHz, (2) observations of the atmosphere and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titan's atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial telescopes, recovering a vertical profile of wind speed in Titan's atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Satumian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, are of fundamental imnortance for the interpretatinn of results from the Huygens mission
Void galaxies
Melkwegstelsels komen voor in gebieden van sterk verschillende dichtheid. Globaal gesproken vindt men melkwegstelsels in "clusters", concentraties van honderden tot duizenden stelsels, in groepen van enkele tot enkele tientallen stelsels, zoals de Lokale Groep waartoe ook onze Melkweg behoort, en in "het veld", zonder een duidelijk verband met andere stelsels. ...
Zie: Samenvatting